Device and method for preparing documents for multicolor reproduction

ABSTRACT

The invention relates to an apparatus and a method for preparing a primary original consisting of picture elements in order to produce processed originals for polychromatic reproduction comprising the following steps: 
     (a) partitioning the primary original into regions which are each assigned to a color class, 
     (b) determining target picture elements within the region of each color class by associating the picture elements to which a source picture element located outside said region is closest according to a predetermined distance measure, 
     (c) labeling non-specific traps in the target picture elements with the color classes of the associated source picture elements and the corresponding distance measures, and 
     (d) generating traps according to predetermined trapping rules by selecting picture elements whose distance measure does not exceed a predetermined trap width, and whose color class determined in step (a), together with the color class of a non-specific trap determined in steps (b) and (c), corresponds to a predetermined trapping rule.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/EP98/05174 which has an Internationalfiling date of Aug. 14, 1998, which designated the United States ofAmerica.

DESCRIPTION

The present invention relates to an apparatus and a method for providingoriginals for polychromatic reproduction, particularly for preparingprinting documents for polychromatic printing.

Printing with four colors which are printed on top of each other iswidely used. In order to produce a spectrum of colors, suitablecoverages of the chromatic primary colors cyan, magenta, and yellow andthe achromatic primary color black are placed successively upon eachimage point of a substrate. Additional colors may be applied, such asgold, silver, lime-green, and others. Mixed colors can be produced bychoosing appropriate amounts of coverage in two or more colors—cyan andyellow, for example, combine to green.

Also used for polychromatic printing are screen printing and so-calledflexographic printing which place several pure colors on a substratesuccessively.

In polychromatic printing the substrate to be printed on, preferablypaper, passes several printing plates successively. The individualprinting plates have to be aligned exactly with the moving substrate forobjects or patterns to be placed in the desired position. In printing,as well as in similar processes for polychromatic reproduction,misregistrations may occur. Misregistrations in printing are attributedto inaccurate guidance of the substrate in the conveying apparatus aswell as in perpendicular direction. It is known to compensate for suchmisregistrations by expanding the colors of individual color areascompletely or partially into adjacent color areas. In this process coloris placed on the substrate beyond the color areas originally provided.This process known as “trapping”, is carried out according topredetermined criteria considering potentially adjoining colors.

In U.S. Pat. No. 5,481,379 a method for preparing polychromatic printingplates is described that is supposed to reduce such misregistrationerrors. In this system, first all pairs of picture elements are examinedin which a color change occurs between adjoining color areas in at leastone color separation. Subsequently, it is determined according tocertain criteria whether a so-called frame should be made around a colorarea and which amount of coverage the frame should have in the differentcolor separations. As examples, trapping rules are given, whichdetermine whether for an existing color pair a trap is generated in thedirection of one color. Furthermore combination rules are given, whichdetermine the trap color of a frame.

In U.S. Pat. No. 5,542,052 a method is described, wherein firstso-called trap areas are generated for individual color areas.Subsequently, parts of the area are removed from these proposed trapareas if they come into undesirably close proximity to other color areasor if they intersect existing trap areas.

It is an objective of the present invention to provide a device and amethod to reduce registration errors in polychromatic reproduction,particularly in polychromatic printing.

This objective is attained by the features set forth in the appendedclaims.

The present invention is based upon the concept of analyzing colorregions rather than boundaries as in the prior art. In a preferredembodiment of the method according to the invention, a distance mappingis carried out in one or a plurality of color regions within which trapsare to be determined.

The invention is hereunder described with reference to examples.Reference will be made to the following drawings:

FIGS. 1a-h show the principle operation of the method according to theinvention wherein all trapping processes of a primary original arecarried out independently.

FIG. 2 shows the principle operation of the alternative method accordingto the invention wherein similar trapping processes are carried outrepetitively.

FIG. 3 is a flow chart of a single trapping process of the principleoperation of the method according to the invention pursuant to FIG. 2.

FIG. 4 shows the generation of traps in the operation of a singletrapping process of FIG. 3.

FIG. 5 shows an example of a primary original.

FIGS. 6a-d show trap determination regions and trap sources for twoexemplary trapping processes of the primary original of FIG. 5.

FIGS. 7a and 7 b compare traps generated with a Euclidean and aorientation-dependent distance measure respectively in a section of theprocessed original of FIG. 9.

FIGS. 8a-f show the generation of traps in a section of the primaryoriginal of FIG. 5.

FIGS. 9a-b show the original of the example of FIG. 5 processed with amethod according to the invention.

FIGS. 10a-g show the basic way of operation of the method according tothe invention in narrow locations of regions within which traps are tobe generated.

FIGS. 11a-c show the basic way of operation of the method of theinvention when trapping color regions comprising a color gradient.

The principle operation of the method of the invention preferablycomprises: partitioning a primary original consisting of pictureelements into different color classes; carrying out independent trappingprocesses within the region of each color class without using specifictrapping rules; and generating traps by selecting picture elementsaccording to predetermined trapping rules.

From a primary original consisting of picture elements being partitionedin the sense of a mathematical definition into color classes it ispreferably formed a number of non-empty, pairwise distinct subregionsconsisting of picture elements, which subregions jointly form theprimary original.

Color classes are regions of picture elements which may be separatedfrom each other, and which are equivalent with respect to one or severaltrapping rules. Other picture elements, preferably picture elementswhich belong to the background of the primary original, may also formcolor classes. Within the color classes, colors are preferablydistinguished. Besides the color thereof, by further preferencetypographic elements of the primary original are distinguished as colorclasses. By still further preference, regions of picture elementsbelonging to logotypes or symbols as well as regions of picture elementsbelonging to halftone images or to predetermined color gradients aredistinguished as color classes.

FIG. 1a shows an example of a partition of a primary original into sixcolor classes 0 to 5.

The trapping processes of all color classes of a primary original arepreferably carried out independently of each other. In the course of onetrapping process non-specific traps of picture elements within theregion of a first color class are generated provisionally; saidnon-specific traps originate from closest picture elements locatedoutside in regions of second color classes and can basically trappicture elements of the first color class. Furthermore, distances of therespective picture elements are derived as values from the correspondingdistance vectors by means of a predetermined mathematical distancemeasure.

FIG. 1b shows the region of color class 3 chosen as an example (depictedin white), and FIG. 1c shows those color classes 0, 1, 2, 4, 5 (alsodepicted white) which basically can trap the chosen color class.

In each independent trapping process of a plurality of first colorclasses, second color classes of picture elements outside the firstcolor classes can be propagated to picture elements within the firstcolor classes as non-specific traps by means of corresponding distancemappings, and distance vectors connecting picture elements of first andsecond color classes can be determined as follows:

Within the region of each color class a distance mapping is carried out.In this connection, distance vectors, the color classes of primaryoriginal forming the target color classes, and the source color classesare assigned to picture elements of the primary original. The sourcecolor classes are empty and all distance vectors carry a predetermineddistance vector expressing a predetermined maximum distance. All pictureelements of the original are visited more than once according to apredetermined scheme. By preference the picture elements are scannedline-by-line first in a top-down scan from the first to the last line,and second in a bottom-up scan from the last to the first line of thepicture. In this process a new distance vector is assigned to eachpicture element if the value of the current distance vector exceeds thevalue of the new distance vector in the sense of a predeterminedmathematical distance measure. The new distance vector is obtained firstby the distance vector pointing to one of its immediate neighbors, ifboth picture element and neighbor are in different target color classes,or second by the sum of the distance vector pointing to one of itsimmediate neighbors and the distance vector of said neighbor if bothpicture element and neighbor are in the same target color class. If thedistance vector of a picture element has been changed, in the first casethe target color class, and in the second case the source color class ofthe immediate neighbor is entered as the source color class to bepropagated of said picture element.

FIG. 1d shows both source and target color classes of non-specific trapsobtained in the process described above. They correspond to theexemplary partition of the primary original into six color classes ofFIG. 1a. The regions of the source color classes are represented bysmall numerals and the regions of the target color classes by largenumerals. The regions of all color classes 0, 1, 2, 3, 4, 5 in theexample are covered by non-specific traps up to a predetermined maximumdistance. By preference the maximum distance for non-specific traps ischosen to correspond to the maximum trap width occurring in apredetermined trapping rule. In the example of FIG. 1d an exaggeratedmaximum distance has been chosen to demonstrate that regions of targetcolor classes can be completely covered by regions of source colorclasses. In the example of FIG. 1d the regions of color classes 2, 3, 4and 5 are completely covered and the regions of color classes 0 and 1are partially covered by regions of source color classes.

Distance vectors between picture elements of target color classes andclosest picture elements of source color classes, also obtained in theprocess described above, are depicted in FIG. 1e by lines of equaldistance up to a predetermined maximum distance. For this example aEuclidean distance measure has been applied. FIG. 1e shows in thesection marked e in FIG. 1d a section of the example where three targetcolor classes 0, 1 and 5 meet.

Traps are generated preferably by selecting picture elements fromnon-specific traps according to predetermined trapping rules. Selectionscan be made according to the corresponding distance vectors as well asthe corresponding source and target color classes of a picture element.

For the selection of picture elements from non-specific traps, thevalues of the corresponding distance vectors can be determined in thesense of a predetermined mathematical distance measure and only thosepicture elements can be selected which do not exceed a predeterminedtrap width. Trap widths can be specified by predetermined trappingrules, preferably for picture elements of all non-specific traps or forpicture elements in regions of predetermined source color classes,predetermined target color classes, or predetermined pairs of source andtarget color classes.

FIG. 1f schematically depicts a set of exemplary trapping rules. Arrowspoint from source color classes to target color classes. In thisexample, the color classes 2 and 5 trap the color class 1; color class 1traps the color class 3; and color classes 1, 2, and 5 trap color class4. Here arbitrary combination rules can be assumed between the color orcolors of picture elements in regions of the corresponding source andtarget color classes. By way of example it is assumed that the color orcolors of the picture elements corresponding to the region of the sourcecolor class result from the combination. FIG. 1f shows, in addition tothe source and target color classes, the trap widths of thecorresponding trapping rules. They are represented schematically by linesegments of different widths. In this example all trap widths are thesame with the exception of traps from color class 5 to color class 1; itis half the size of the other trap widths.

Furthermore, for the selection of picture elements from non-specifictraps, the corresponding source and target color classes can be examinedin order to reduce the selection to only those picture elements thepairs of source and target color classes of which correspond to a pairof source and target color classes of a certain trapping rule.

FIG. 1g for example shows traps which have been prepared from thenon-specific traps in FIG. 1d according to the specifications oftrapping rules in FIG. 1f. The second trapping rule in FIG. 1fdetermines that color class 5 traps color class 1 with half of apredetermined trap width. Selected from the non-specific traps in FIG.1d are picture elements of source color class 5 and target color class1, whose distance vectors have values in the sense of a mathematicaldistance function which do not exceed half of the predetermined trapwidth. FIG. 1 g shows them as narrow traps in the region of target colorclass 1. The remaining trapping rules in FIG. 1f state that color class2 traps color class 1, color class 1 traps color class 3, and the colorclasses 1, 2, 3 and 5 trap color class 4 with the full trap width.Selected from non-specific traps in FIG. 1d are picture elements with(source, target) pairs of color classes (2,1), (1,3), (1,4), (2,4),(3,4) and (5,4) whose distance values do not exceed the fullpredetermined trap width. FIG. 1g shows them as traps in the regions ofthe target color classes 3 and 4.

The generation of traps by a selection of picture elements fromnon-specific traps can be carried out by a selection according to pairsof color classes and distance values combined, as described by theexample above, or separately according to pairs of color classes anddistance values.

It should be stated explicitly that the method according to theinvention is not limited to any particular set of trapping rules. Adifferent set of trapping rules as that given in the example of FIG. 1frather would determine other traps and hence lead to different results.

The invention can be applied to originals which consist in whole or inpart of geometric representations comprising line or curve segments(vector representations) for the reproduction of graphical objectsparticularly on cathode ray tubes or in polychromatic printing. A methodof known art can be used for converting vector representations to rasterrepresentations, preferably to matrices of picture elements.

Furthermore, according to the invention in a plurality of pictureelements of the primary original pairs of color classes of non-specifictraps can be selected as traps directly, according to predeterminedtrapping rules, particularly after selection according to distancevalues.

In order to select pairs of color classes and the corresponding pictureelements indirectly, particularly after a selection according todistance values, according to a preferred embodiment of the methodaccording to the invention geometric representations are used whichrepresent (vectorize) picture elements corresponding to identical pairsof source and target color classes by their contours. In particular, aprocess of the applicant can be used which is distributed under theregistered (DE) trademark Vectory.

According to an alternative embodiment of the method according to theinvention, traps extending in both directions of adjacent color classescan be generated from non-specific traps, particularly after selectionaccording to distance values and furthermore for generating trapsaccording to predetermined trapping rules which contain each pair ofcolor classes at least once. This is attained by jointly selectingpicture elements of non-specific traps with symmetric pairs of colorclasses, e.g. (1,2) and (2,1), for traps in both directions of thecorresponding pair of color classes, if one of the correspondingsymmetric pairs of color classes appears in a predetermined trappingrule. For the exemplary trapping rules of FIG. 1f these are the colorpairs (2,1), (5,1), (1,3), (1,4), (2,4), (3,4) and (5,4). The resultingtraps extend in both directions joining the regions of the followingcolor classes: 2 and 1; 5 and 1; 1 and 3; 1 and 4; 2 and 4; 3 and 4 aswell as 5 and 4. They are shown in FIG. 1h. Here it is assumed thatpicture elements are selected beforehand from the non-specific traps inFIG. 1d by distance values according to the trap widths specified in thetrapping rules determined by way of example in FIG. 1f.

As the result of the combination of the colors in the respective sourceand target color classes of the thus generated traps extending in bothdirections, a color can be generated which does not change the color ofthe source color class in combination with said source color class,preferably by overprinting, and, in combination with the color of thetarget color class, it yields the trap color determined in thecorresponding trapping rule, preferably by overprinting. Here the colorof the source color class was taken as the result of the correspondingtrapping rule.

A preferred application of traps extending in both directions is thesubsequent reversal of the direction of traps with minimal effort. Thiscan be attained by simply changing the combination color. In the exampleabove a reversal of the trap direction is attained by replacing thecolor of the source color class by the color of the target color classas the combination result of the corresponding trapping rule.

Vectorized representations of traps extending in both directions can begenerated with the invention preferably by using a vectorization methodof known art.

In a preferred embodiment of the method according to the invention,results can be generated in raster form or vector form or in raster formregenerated from vector form and can be appended to or combined with theprimary original in raster or vector form.

FIG. 2 shows the principle operation of the alternative method accordingto the invention wherein similar single trapping processes are carriedout repetitively. For one or a plurality of first similar color regionsa trapping process 160 is carried out using the primary original 110.The trapping process is repeated with one or more similar color regionsby looping back 180 until all similar color regions of the primaryoriginal 110 are processed and the processed original 190 can be output.

One or a plurality of color regions, which may be separated from eachother and have identical or similar color characteristics, areconsidered as a region of similar color. Regions of picture elementswith identical colors are considered as having identical colorcharacteristics. All other predetermined regions, including regionscomprising color gradients, regions comprising halftone images orsubregions of halftone images as well as subregions of the primaryoriginal 110 selected by the user are considered as having similar colorcharacteristics.

The following refers to FIG. 3. In a single trapping process 160, a trapdetermination region is formed 210, by selecting one or a plurality ofsimilar color regions, which may be separated from each other, from theprimary original 110. Within said trap determination region traps ofcolors in adjacent color regions are to be determined. Picture elementsof the primary original 110 or of an intermediate result of theprocessed original 190, are determined as trap source in step 220; theircolor may trap the color or colors of the picture elements in the trapdetermination region according to predetermined trapping rules.

The trapping rules 230 can be predetermined in data sets generally orindividually for certain primary originals, or they can be computed fromcolor pairs.

For further explanation of the generation of traps 250 within the trapdetermination region according to a preferred embodiment of the methodaccording to the invention, an exemplary primary original 110 shall beconsidered in FIG. 5. It shows a black square 410 which is partiallycovered by a cyan triangle 420, and both are on a yellow background 490.

In the preferred embodiment of the method according to the presentinvention described below, simple trapping rules 230 are assumed forapplication in the individual trapping processes 160. A set of trappingrules is obtained for example by determining or having determined foreach color in the primary original 110 which other colors it can betrapped with, including the possibility that it is not trapped by anycolor or it is trapped by one color only.

The color pairs mentioned above can be determined for instance byassuming that the darker of two colors is trapped by the lighter color.Of the colors in the primary original 110 in FIG. 5 black is thedarkest, cyan is of medium lightness, and yellow is the lightest color.Yellow is not trapped by any color, cyan is trapped by yellow, and blackis trapped by all other colors. In this example, wherever two differentcolors abut, the darker color is trapped by the lighter color. The twooverlaying colors reproduced in the same location produce a trap color.

Contrary to known methods of prior art, this alternative methodaccording to the invention disclosed below does not compare any adjacentpicture elements in order to decide whether they belong to two adjacentregions. Besides the sometimes considerable savings in computation loadit is irrelevant for the determination of color pairs as trapping rulesexplained above whether a given color pair definitely occurs in twoadjacent picture elements of the primary original 110.

A preferred embodiment of the method according to the invention rathergenerates all traps simultaneously which trap a certain region in thesense described above as limited extensions of the correspondingadjacent regions.

The following explanation of the concept of the invention refers toFIGS. 3 and 4. The individual trapping processes result from thoseregions of similar color in the primary original 110 which are trappedby other colors according to predetermined trapping rules 230. For theprimary original 110 the exemplary trapping rules 230 yield that cyan istrapped by yellow in the first trapping process, and that black istrapped by cyan and yellow in the second trapping process. The sequenceof the trapping processes, i.e. the order of repeated executions of thesequence of instructions 160 is chosen arbitrarily.

For an explanation of a trapping process, attention is drawn to the flowchart of FIG. 3. After a trap determination region has been formed 210,and a trap source has been determined 220 according to the trappingrules 230, all traps are formed in the sequence of instructions 250 fromthe trap source into the trap determination region. Subsequently step180 (see FIG. 2) is executed.

It should be stated explicitly that the method according to theinvention is not limited to any particular set of trapping rules 230.Predetermining a different set of trapping rules 230 rather would yielddifferent trap sources 220 and hence lead to different results in theprocessed original 190.

FIG. 8 shows the generation of traps in the section of the primaryoriginal 110 marked 480 in FIG. 5.

The primary original 110 contains three colors: yellow, cyan and black.It shall be assumed that each color has its own color separation andthat each color is reproduced separately, e.g. is printed separately.According to the simple trapping rules: (i) cyan is trapped by yellow,and (ii) black is trapped by yellow and cyan, the traps are generated intwo trapping processes. Because there are only two color regions of thecolors cyan and black in the primary original 110, traps of cyan aregenerated in the first trapping process, and traps of black aregenerated in the second trapping process.

FIGS. 8a and 8 d show trap determination regions during the first andsecond trapping process, respectively, in the section 480 of the primaryoriginal 110. FIGS. 8b and 8 e show the corresponding trap sources.Finally, FIGS. 8d and 8 f show the traps which are generated in thefirst and second trapping process.

The trap determination region of the first trapping process comprisesall picture elements which carry the color cyan. A distance mapping ofsaid trap determination region is carried out in one or more auxiliarycolor channels.

The region of the distance mapping is shown as a cutout in FIG. 8a. Forreference, it is shown as white region in FIG. 6a. A distance mappingspecifies for each picture element in the map region the distance to aclosest picture element outside the map region.

For clarity, FIG. 8a shows distances up to an assumed trap width of sixpixels only. A pixel represents the width of a picture element as a unitof length. Those target picture elements which share the same closestsource picture element outside the trap determination region are shownin FIG. 8a connected by sequences of line segments. Source pictureelements are depicted by white dots. The distances themselves are notshown.

The trap source comprises all picture elements the color of which trapsthe color or colors of picture elements in the trap determinationregion. In the first trapping process of the present example, they aredetermined by the color separation of the color yellow. The trap sourceis shown as a cutout in FIG. 8b and for reference in FIG. 6b. In thefirst trapping process traps are rendered in the color separation of thecolor yellow.

The following discussion of further details of the generation of traps250 refers to the flow chart of FIG. 4. If it is not determined ininstruction step 320 that the trap determination region is empty, afirst target picture element in the trap determination region isselected provisionally in instruction step 330. The distance from saidtarget picture element to a closest source picture element outside thetrap determination region is determined in instruction step 340,preferably by “look-up” in a distance mapping of the trap determinationregion. If said distance is less than or equal to the predetermined trapwidth, the next sequence of instructions 360 is executed. Alternativelyor after execution of the sequence of instructions 360 the branchingstep 320 is executed again. It is repeated together with the instructionsteps 330, 340, 350 and 360 if necessary until all picture elements ofthe trap determination region are processed and control can return tothe end of the sequence of instructions 250 by means of the instruction320. In the sequence of instructions 360 the separation values (thecoverage values of the color separations) of the target picture elementare combined with the separation values of the source picture element inthose color separations in which the trap is to be rendered. In thefirst trapping process of the present example the separation value ofthe color yellow is transferred from the source picture element to thetarget picture element (see FIG. 8c).

The trap determination region of the second trapping process comprisesall picture elements which carry the color black. A distance mapping ofsaid trap determination region is carried out in one or more auxiliarycolor channels.

The trap determination region of the second trapping process is shown asa cutout in FIG. 8d. For reference, it is shown as white region in FIG.6c. Again distances are considered up to an assumed trap width of sixpixels only.

The trap source comprises all picture elements the color of which trapsthe color or colors of the picture elements in the trap determinationregion. In the second trapping process of the present example they aredetermined by the color separations of the colors yellow and cyan. Thetrap source is shown as a cutout in FIG. 8e and for reference in FIG.6d. FIG. 8e also shows the trap of yellow by cyan already generated inthe first trapping process. The reproduction of the superimposed colorsyellow and cyan in the corresponding area produces, for example, a greencolor.

In the second trapping process the traps are rendered in the colorseparations of the colors yellow and cyan. In each picture element ofthe trap determination region, to which a closest picture elementoutside the trap determination region is closer than a predeterminedtrap width, the separation values of the trap source of said closestpicture element are entered (see FIG. 8f). The reproduction of thesuperimposed colors in this area produces for instance a black colorwhich is oversaturated with yellow and cyan.

An alternative way to specify the trap source of a trapping process 160is for example to provide a mask which determines for each pictureelement which of the following separation values should be used as trapsource: either the constant separation values for zero coverage or theseparation values of the primary original 110 or an intermediate result.In instruction step 360 the simple combination rule of transferring theseparation values from source to target picture elements can be replacedfor instance by a combination rule which takes the maximum values ineach separation of source and target picture elements as the separationvalues of the corresponding target picture element.

FIG. 9 shows the original 190 of the present example processed accordingto the trapping method of invention: all color separations yellow, cyan,and black are shown in superposition including traps. The separation ofthe color black has not been changed. In FIGS. 9a and 9 b the colorregions contained in the separations of the color yellow and cyanrespectively are emphasized by framing.

The preceding example has been described by means of colors inindividual separations for the colors yellow, cyan, and black,respectively. As it has been outlined, arbitrary colors may be used. Themethod is generally applicable.

As a further example, colors with separation values according to thefollowing table are chosen for the regions 490, 420 and 410 of theprimary original 110:

490 420 410 C 20 60 80 M 60 20 50 Y 60 60 30 K 10 10 10.

Cyan, magenta, yellow, and black are denoted C, M, Y and K,respectively. As a first trapping rule, it is assumed that the color ofthe region 420 is trapped by the color of the region 490 and that bothcolors are combined to the trap color designated U1 in the followingtable:

420 490 U1 C 60 20 60 M 20 60 60 Y 60 60 60 K 10 10 10.

As second and third trapping rules, it is assumed that the color of theregion 410 is trapped by the color of the region 420, resulting in thetrap color U2, as well as by the color of the region 490, resulting inthe trap color U3 as listed in the following tables:

410 420 U2 C 80 60 80 M 50 20 50 Y 30 60 60 K 10 10 10 and 410 490 U3 C80 20 80 M 50 60 60 Y 30 60 60 K 10 10 10.

The first trapping rule determines the traps in the region of the color420, the first trap determination region in FIG. 6a. The second andthird trapping rule determine the traps in the region of the color 410,the second trap determination region in FIG. 6b.

FIG. 7a illustrates in a section of the processed original in FIG. 9 aportion of the trap of the cyan triangle 420 into the black square 410.In the distance measurement of instruction step 340 (see FIG. 4) aEuclidean distance measure was used wherein the distance between twopicture elements measuring for example three lines and four columns ofthe picture is evaluated as follows: 5=square root of (3²+4²). TheEuclidean distance measure is used preferably to generate traps whichspread uniformly in all directions.

FIG. 7b shows in the same section of FIG. 9 a part of a trap, which hasbeen generated with a orientation-dependent distance measure. Asindicated by the direction arrows, said trap spreads non-uniformly indifferent directions. Orientation-dependent distance measures are usedpreferably to reduce registration errors in polychromatic reproductionwhich are also orientation-dependent, wherein the correction indirections where only no or minor misregistrations are to be expected isas low as possible.

By preference the generation 250 of traps in a trap determination regionis carried out by means of a distance mapping. A distance mapping isimplemented as follows:

A distance mapping is carried out in an original comprising pictureelements associated with distance vectors. Initially, a vanishingdistance vector is assigned to picture elements outside the map region,and a predetermined distance vector expressing a predetermined maximumdistance is assigned to picture elements inside the map region.

All picture elements of the original are visited more than onceaccording to a certain scheme. In this process a new distance vector isassigned to each picture element, said distance vector is determined bythe sum of the distance vector directed to one of its immediateneighbors and the distance vector of that neighbor, provided that thevalue of the sum vector is less than the value of the old distancevector in the sense of a predetermined mathematical distance measure.

A description of a certain method of distance mapping is given forexample in a paper by P.-E. Danielsson entitled: “Euclidean DistanceMapping”, which is published in the journal “Computer Graphics and ImageProcessing”, volume 14, page 227-248 (1980).

By means of a distance mapping color values corresponding to pictureelements outside the trap determination region can be propagated astraps onto picture elements inside the trap determination region, asfollows:

A distance mapping is carried out in an original comprising pictureelements associated with distance vectors and predetermined color valuesto be propagated. Initially, a vanishing distance vector and apredetermined color value to be propagated are assigned to pictureelements outside the trap determination region, and a predetermineddistance vector expressing a predetermined maximum distance, andpreferably a neutral color value to be propagated are assigned topicture elements inside the trap determination region.

All picture elements of the original are visited more than onceaccording to a certain scheme. In this process a new distance vectordetermined by the sum of the distance vector directed to one of itsimmediate neighbors and the distance vector of that neighbor, providedthat the value of the sum vector is less than the value of the olddistance vector in the sense of a predetermined mathematical distancemeasure, is assigned to each picture element. In this case the colorvalue to be propagated of said immediate neighbor is combined with theold color value to be propagated of said picture element to a new colorvalue to be propagated of said picture element.

An example of the propagation of color values as described above isshown in FIGS. 8c and 8 f including “dentrites” pointing from sourcepicture elements of a trap source to target picture elements in a trapdetermination region.

Narrow locations of areas in which traps are to be generated often poseproblems. They include the following effects (see FIGS. 10a to 10 c):(1) a trap 801 from an area 800 into an area 810 intersects aneighboring area 820 causing an undesired overlap in the reproduction819; (2) traps 802 and 821 from different areas 800 and 820 intersecteach other causing an undesired overlap in the reproduction 829; a trap803 from an area 800 into an area 810 is too close to a neighboring area820 and stands out in case of minor misregistrations in the reproductionof this area 829. The mentioned effects occur in primitive methods whichgenerate traps assuming an undisturbed course of said traps.

These effects are largely prevented if precautions are taken that trapsdo not cross the median of narrow locations. Thus traps are notimmediately prominent in the reproduction in the event ofmisregistrations and resist prominence uniformly in all directions tomaximum degree. FIG. 10d shows a section of the distance mapping in thetrap determination region 860 which belongs to the examples in FIGS. 10ato 10 c. The distance map is depicted schematically by “lines of slope”850 (lines of maximal change in distance as the “dendrites” in FIG. 8before) without restriction to a particular trap width. The lines ofslope point from picture elements of the trap determination region 860to closest elements outside the trap region 870. Traps generatedaccording to a preferred embodiment of the method according to thepresent invention always follow the restriction not to cross the medianor, expressed more illustratively, the “distance sheds” of a trapdetermination region. This property is independent of the predeterminedtrap width or trap widths 851, 852, 853. Said property is furtherindependent of the occurrence of traps in narrow locations either assingle 851, 853 or multiple traps 852 and 871.

Methods according to prior art which first generate traps assuming anundisturbed course of said traps, and consider traps already generatedor neighboring or proximate areas of the original separately, have toadapt or change the shapes of the primitive traps often withconsiderable effort in order to obtain for instance the desirable courseof traps described above.

The method according to the present invention generates all traps of aprimary original without comparing any traps with each other or withcolor regions of the primary original.

Color gradients present a further problem for trapping. For traps ofcolor regions comprising a color gradient, it is important to smoothlyblend the color gradient into the trap. With methods according to priorart said traps often can be generated with considerable processingeffort only.

FIG. 11 shows the basic operation of the method according to the presentinvention for trapping color regions comprising a color gradient. InFIG. 11a the distance mapping of a trap determination region 900 isdepicted again by “lines of slope” 910. They point from picture elementsof the trap determination region 900 to closest elements outside thetrap region 920. In an adjacent trap source 930 wherein a linear colorgradient is assumed, picture elements of identical color are depictedschematically by lines of identical width 940. For traps generatedaccording to a preferred embodiment of the method according to thepresent invention, different colors 940, 950 of a gradient in a trapsource 930 always follow the “lines of slope” 910 of a trapdetermination region 900 and thus propagate a color gradient smoothly960, 970.

The method according to the present invention generates traps of colorregions with color gradients without any additional effort compared totraps of simple color regions with “flat” colors.

According to a further preferred embodiment of the method according tothe present invention, the distance between target and source pictureelements is considered in the combination of the separation values ininstruction step 360 (see FIG. 4), for instance in order to reduce thepropagation of traps.

If a primary original 110 contains text elements, then by preferencedifferent trapping rules are applied for the text elements and for theremaining parts of the primary original, e.g. to minimize distortions ofthe forms of letters.

Situations arise in which traps generated according to the presentinvention are stored preferably as intermediate results. Thus it isprevented that separation values of the primary original which areneeded for other traps are overwritten. This happens for instance ingenerating traps which have to be propagated into two abutting colorregions, particularly if both color regions comprise color gradients.

The invention can be combined with an image processing system of knownart, wherein the method according to the present invention can be usedas a so-called program extension or “plug-in”.

With the invention, originals can be prepared which serve directly orindirectly as printing documents for polychromatic printing.

In a preferred embodiment of the present invention, the primary originalis first split into partial areas, preferably bands or tiles, whereinthe partial areas overlap each other to prevent boundary errors.Subsequently, each primary partial area is processed individually andthereafter combined with the remaining processed partial areas to acompletely processed original.

An apparatus according to the present invention for preparing a primaryoriginal comprising picture elements by preference comprises a rasterimage processor (RIP) for processing and a frame buffer for storing theprimary original, as well as auxiliary color channels. Both units areused for instance in electronic prepress equipment.

In order to control the devices of the apparatus according to thepresent invention, by preference a computing program is executed on asuitable raster image processor. Particularly, this program can beexpressed in a page description language, e.g. in PostScript (PostScriptis a registered trademark of Adobe Systems Inc. in San Jose, Calif.95110, USA).

What is claimed is:
 1. A method for preparing a primary originalconsisting of picture elements for producing processed originals forpolychromatic reproduction, comprising the following steps: (a)partitioning the primary original into regions which are each assignedto a color class, (b) determining target picture elements within theregion of each color class by associating the picture elements to whicha source picture element located outside said region is closestaccording to a predetermined distance measure, (c) labeling non-specifictraps in the target picture elements with the color classes of theassociated source picture elements and the corresponding distancemeasures, and (d) generating traps according to predetermined trappingrules by selecting picture elements whose distance measure does notexceed a predetermined trap width and whose color class determined instep (a), together with the color class of a non-specific trapdetermined in steps (b) and (c), corresponds to a predetermined trappingrule.
 2. The method of claim 1, wherein the association of the pictureelements in step (b) is carried out for each region by means of acorresponding distance mapping.
 3. The method of claim 1 or 2, whereinthe primary original is present in vector form or in mixed raster orvector form, and is converted to raster form, preferably to a matrix ofpicture elements, prior to performing steps (a) to (d).
 4. The method ofclaims 1 to 3, wherein in step (d), particularly after the selectionaccording to distance values, the selection according to pairs of colorclasses is carried out for picture elements of the primary original. 5.The method of claims 1 to 3, wherein in step (d), particularly after theselection according to distance values, the selection according to pairsof color classes is carried out in a vectorized representation,preferably in a geometric representation of the outlines of pictureelements which belong to identical pairs of color classes.
 6. The methodof claims 1 to 3 wherein in step (d) traps extending in both directionstowards adjacent color classes are generated.
 7. The method of claim 6,wherein a vectorized representation of the traps in both directions isperformed.
 8. The method of claim 4 or 6, wherein the traps in rasterform are appended to or combined with the primary original in raster orvector form.
 9. The method of claim 5 or 7, wherein the traps invectorized form are appended to or combined with the primary original inraster or vector form.
 10. The method of claim 5 or 7, wherein the trapsin vectorized form are converted to raster form and appended to orcombined with the primary original in raster or vector form.
 11. Themethod, in particular according to claim 1, for preparing a primaryoriginal consisting of picture elements and further comprising aplurality of color regions, wherein similar color regions may beseparated from each other, for producing processed originals forpolychromatic reproduction, comprising the following steps: (a) forminga trap determination region comprising one or a plurality of firstsimilar color regions, (b) determining the trap source which comprisespicture elements the color of which can trap said first similar colorregions according to predetermined trapping rules, (c) determiningtarget picture elements within the trap determination region byassociating the picture elements to which a source picture elementlocated outside the trap determination region is closest according to apredetermined distance measure and which is located within apredetermined trap width, and (d) combining the colors of the targetpicture elements with the colors of the associated source pictureelements according to a combination rule, wherein the trapping processcomprising steps (a) to (d) may be repeated for further regions ofsimilar color and the corresponding trap sources.
 12. The method ofclaim 11, wherein in step (c) the association of picture elements iscarried out by means of a distance mapping.
 13. The method according toany one of claims 1 to 12, wherein each color or color class isassociated with at least one color separation.
 14. The method of claim13 wherein in step (d) the separation values of the target pictureelements are combined with the separation values of the source pictureelements.
 15. The method according to any one of claims 1 to 14, whereinthe distance measure is a mathematical distance function, preferably aEuclidean distance measure.
 16. The method according to any one ofclaims 1 to 15, wherein the distance measure is orientation-dependent.17. The method according to any one of claims 1 to 16, wherein differentcombination rules are applied for the combination of separation valuesof target picture elements with separation values of the associatedsource picture elements, depending on the distance between said targetand source picture elements.
 18. The method of claim 17 wherein thecombination rule is varied such that, with increasing distance of atarget picture element to a source picture element, the contribution ofthe associated source picture element to the amount of coverage in colorseparations decreases.
 19. The method according to any one of claims 1to 18, wherein an original comprises a plurality of regions for whichdifferent trapping rules apply.
 20. The method according to any one ofclaims 1 to 19, wherein traps of one or more trapping processes arestored as intermediate results and combined with other trap results orintermediate results.
 21. The method according to any one of claims 1 to20, wherein the method is carried out in an image processing system ofknown art.
 22. The method according to any one of claims 1 to 21,wherein the processed originals are used for producing printingdocuments for polychromatic printing.
 23. The method according to anyone of claims 1 to 22, wherein the method is applied to partial areas,preferably bands or tiles, which are obtained by partitioning theprimary original, and wherein each partitioned primary area is processedseparately and thereafter combined with the remaining partiallyprocessed areas to a completely processed original.
 24. An apparatus forpreparing a primary original consisting of picture elements in order toproduce processed originals for polychromatic reproduction, comprising:(a) a first device for partitioning the primary original into regionswhich are each assigned to a color class, (b) a second device fordetermining target picture elements within the region of each colorclass by associating the picture elements to which a source pictureelement located outside said region is closest according to apredetermined distance measure, (c) a third device for labelingnon-specific traps in the target picture elements with the color classesof the associated source picture elements and the corresponding distancemeasures, and (d) a fourth device for generating traps according topredetermined trapping rules by selecting picture elements whosedistance measure does not exceed a predetermined trap width and whosecolor class determined in step (a), together with the color class of anon-specific trap determined in steps (b) and (c), corresponds to apredetermined trapping rule.
 25. The apparatus, in particular accordingto claim 24, for preparing a primary original consisting of pictureelements and further comprising a plurality of color regions, whereinsimilar color regions may be separated from each other, for producingprocessed originals for polychromatic reproduction, comprising: (a) afirst device for forming a trap determination region comprising one or aplurality of first similar color regions, (b) a second device fordetermining the trap source which comprises picture elements, the colorof which can trap said first similar color regions according topredetermined trapping rules, (c) a third device for determining targetpicture elements within the trap determination region by associating thepicture elements to which a source picture element located outside thetrap determination region is closest according to a predetermineddistance measure, and which is located within a predetermined trapwidth, and (d) a fourth device for combining the colors of the targetpicture elements with the colors of the associated source pictureelements according to a combination rule.
 26. The apparatus of claim 24or 25, wherein the first to the fourth device are provided by means of araster image processor (RIP) which preferably has at least one framebuffer.
 27. Apparatus according to any one of claims 24 to 26, whereinthe devices and the raster image processor, respectively, are controlledaccording to the methods of any one of claims 1 to 23.